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Tetrachromacy is a condition of having four colour channels. There is a hypothesis that some women posses 4 different types of cone cells (each with different opsin responsible for different absorption spectrum) and can process them.

What is the current status of tetrachromacy in humans? Is it sufficiently confirmed by the experiment, refuted or unresolved?

I am aware that among medical researchers positive 'deviations' can attract less attention than deficits or diseases. Moreover, they may be more prone to methodological mistakes as it is easier to measure deficits against standard criteria.

1 Answer
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Jordan et al. (2010) tested humans that carry the genetic predisposition for having 4 classes of cone receptors. However, only one of their 24 subjects was able to discriminate three color dimensions, thus, demonstrated tetrachromacy.

Some 12% of women are carriers of the mild, X-linked forms of color vision deficiencies called "anomalous trichromacy." Owing to random X chromosome inactivation, their retinae must contain four classes of cone rather than the normal three; and it has previously been speculated that these female carriers might be tetrachromatic, capable of discriminating spectral stimuli that are indistinguishable to the normal trichromat. However, the existing evidence is sparse and inconclusive. Here, we address the question using (a) a forced-choice version of the Rayleigh test, (b) a test using multidimensional scaling to reveal directly the dimensionality of the participants' color space, and (c) molecular genetic analyses to estimate the X-linked cone peak sensitivities of a selected sample of strong candidates for tetrachromacy. Our results suggest that most carriers of color anomaly do not exhibit four-dimensional color vision, and so we believe that anomalous trichromacy is unlikely to be maintained by an advantage to the carriers in discriminating colors. However, 1 of 24 obligate carriers of deuteranomaly exhibited tetrachromatic behavior on all our tests; this participant has three well-separated cone photopigments in the long-wave spectral region in addition to her short-wave cone. We assess the likelihood that behavioral tetrachromacy exists in the human population.